This invention relates to fibers and yarns made from poly (paraphenylene terephthalamide) (PPD-T) that exhibit high initial strength and retain a high percentage of that high strength after a timed exposure to certain high temperatures. These fibers and yarns are useful in fiber applications wherein the fibers and/or yarns are exposed to high temperatures during later processing, such as in tire manufacture. For example, it is important that any cords that include such fibers and/or yarns do not lose substantial strength during exposure to high temperatures during rubber curing steps.
U.S. Pat. Nos. 5,182, 067 & 5,302,451 to Chiou discloses as-spun fiber having 0.5 to 3.0 percent sulfur as bound sulfonic acid or sulfonate groups. The fiber and has an as-spun yarn tenacity greater than 20 grams per denier, heat-aged strength retention of greater than 90 percent, and dipped cord strength greater than 18 gram per denier. The process for making the fiber includes PPD-T polymer sulfonation by exposure to highly concentrated sulfuric acid in dope preparation under carefully controlled temperature and time.
U.S. Pat. Nos. 3,767,756; 3,869,429; & 3,869,430 to Blades disclose fibers of at least about 18 gpd consisting essentially of certain polyamides. U.S. Pat. No. 3,767,756 discloses heat treatment of the as-spun fibers, preferably in an inert atmosphere, to provide filaments having a much higher modulus but lower breaking elongation. U.S. Pat. Nos. 3,869,430 & 3,869,429 describe a heat treatment process for as-spun PPD-T filaments such as those prepared according to the process of U.S. Pat. No. 3,767,756 whereby the filaments are heated under a tension of at least 0.5 gpd (0.45 g/dtex) but less than the tension required to draw the filaments more than 1.03 times their initial length. The preferred temperature of the heating zone is 250-600° C., most preferably 450-580° C. These products have filament elongations of at least 3.5 percent and possess a novel crystalline structure with crystalline regions having a primary apparent crystallite size of less than 52 Angstrom units.
U.S. Pat. Nos. 4,374,978 & 4,440,710 to Fujiwara et al. disclose a high Young's modulus PPD-T fiber made by washing and drying fibers in the absence of substantial tension and heating the fibers under tension. Fujiwara et al. discloses the fibers of U.S. Pat. Nos. 4,374,978 & 4,440,710 have a larger orientation angle (OA) than fibers disclosed in the previously mentioned U.S. Pat. No. 3,869,430, etc. to Blades, and this relatively large orientation angle is closely related to the manufacturing process wherein all the steps from washing to drying are carried out in the absence of tension and the heat treatment is conducted under tension.
U.S. Pat. Nos. 4,859,393 & 4,902,774 disclose a PPD-T yarn having an apparent crystallite size in the range of 40 to 50 A, an orientation angle in the range of 20° to 30°, an elongation in the range of 4.5 to 5.6 percent, a tenacity of at least 18 grams per denier, and a modulus of at least 200 grams per denier and less than 450 grams per denier. These patents further disclose a process wherein a polymer is spun through an air gap into a coagulating bath at a temperature of at least about 20° C., but not greater than 40° C., and removed from the bath, the improvement consisting of washing the yarn and neutralizing the acid therein while the fiber is under a tension in the range of 0.2 to 0.4 grams per denier and then drying the yarn at a temperature below 200° C. under a tension in the range of from 0.05 to 0.2 grams per denier.
U.S. Pat. No. 5,173,236 to Yang discloses a process for making PPD-T fiber having increased tenacity and elongation at break obtained by the following combination of steps: a) extruding the anisotropic spinning dope through a capillary having a diameter of less than 64 micrometers (2.5 mils); b) maintaining the coagulation bath at a temperature of less than 10° C.; and c) washing and drying the coagulated fiber at controlled, substantially constant, tensions of 0.05 to 0.35 gpd, preferably 0.05 to 0.25 gpd. All of the aforementioned combination of steps must be used in order to realize the improvement of this invention.
U.S. Pat No. 4,726,922 to Cochran & Yang disclose a process for obtaining PPD-T filaments having improved tenacity by drying the filaments at a temperature of less than 300° C. under a tension of at least 2 gpd and thereafter discontinuing the drying under tension while the filaments on heated rolls have a moisture content of at least 8 percent.
U.S. Pat. No. 4,320,081 to Lammers discloses a process for the manufacture of fibers wherein the coagulation bath temperature is generally in the range of −10° to 50° C., and preferably between 0 and 25° C. The examples of this patent further disclose drying the fibers at temperatures of 120° and 140° C.
In many industrial applications, there is a need for fibers that exhibit a high strength when new, while at the same time retaining a high strength after exposure to certain extreme conditions, which can include elevated thermal conditions (high temperatures). In applications such as in tire walls, hoses, or belting, etc., one of the most important fiber strength qualities is the measured yarn strength retained after the cords containing those yarns are incorporated into the various finished elastomeric articles. In some instances, the yarns are formed into cords that are subsequently processed into dipped cords. In many instances, these are cords made from yarns that been coated with polymeric materials designed to increase adhesion of the yarn and/or cord to matrices such as rubber. Useful fibers retain a high strength after exposure to high temperatures during processing of the yarns into dipped cords, and/or further processing of yarns and/or cords into the finished elastomeric product, which can include exposure to high elastomeric and/or rubber curing temperatures.
Therefore, one key measured value for such yarns prior to being dipped is the property of “strength retention”, specifically “heat-aged strength retention” (HASR). It is believed that the apparent crystallite size (ACS) is one important property related to improving HASR. Unfortunately, prior processes that teach methods for increasing the ACS in the fiber also result in increased yarn tensile modulus along with decreased yarn tenacity. Further, a yarn having increased tensile modulus is undesirable in some applications because it is believed the increased stiffness of the yarn contributes to increased compression fatigue.
Therefore what is needed is a process for making a new PPD-T yarn, and the yarn made thereby, the yarn having increased crystallinity as measured by apparent crystallite size (ACS) but also having the other favorable attributes desired in the yarn, such as high tenacity, moderate to lower modulus, and high elongation at break.